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    <h1 align="CENTER">Descriptors and Apache</h1>

    <p>A <em>descriptor</em>, also commonly called a <em>file
    handle</em> is an object that a program uses to read or write
    an open file, or open network socket, or a variety of other
    devices. It is represented by an integer, and you may be
    familiar with <code>stdin</code>, <code>stdout</code>, and
    <code>stderr</code> which are descriptors 0, 1, and 2
    respectively. Apache needs a descriptor for each log file, plus
    one for each network socket that it listens on, plus a handful
    of others. Libraries that Apache uses may also require
    descriptors. Normal programs don't open up many descriptors at
    all, and so there are some latent problems that you may
    experience should you start running Apache with many
    descriptors (<em>i.e.</em>, with many virtual hosts).</p>

    <p>The operating system enforces a limit on the number of
    descriptors that a program can have open at a time. There are
    typically three limits involved here. One is a kernel
    limitation, depending on your operating system you will either
    be able to tune the number of descriptors available to higher
    numbers (this is frequently called <em>FD_SETSIZE</em>). Or you
    may be stuck with a (relatively) low amount. The second limit
    is called the <em>hard resource</em> limit, and it is sometimes
    set by root in an obscure operating system file, but frequently
    is the same as the kernel limit. The third limit is called the
    <em>soft resource</em> limit. The soft limit is always less
    than or equal to the hard limit. For example, the hard limit
    may be 1024, but the soft limit only 64. Any user can raise
    their soft limit up to the hard limit. Root can raise the hard
    limit up to the system maximum limit. The soft limit is the
    actual limit that is used when enforcing the maximum number of
    files a process can have open.</p>

    <p>To summarize:</p>

    <center>
<pre>
  #open files  &lt;=  soft limit  &lt;=  hard limit  &lt;=  kernel limit
</pre>
    </center>

    <p>You control the hard and soft limits using the
    <code>limit</code> (csh) or <code>ulimit</code> (sh)
    directives. See the respective man pages for more information.
    For example you can probably use <code>ulimit -n
    unlimited</code> to raise your soft limit up to the hard limit.
    You should include this command in a shell script which starts
    your webserver.</p>

    <p>Unfortunately, it's not always this simple. As mentioned
    above, you will probably run into some system limitations that
    will need to be worked around somehow. Work was done in version
    1.2.1 to improve the situation somewhat. Here is a partial list
    of systems and workarounds (assuming you are using 1.2.1 or
    later):</p>

    <dl>
      <dt><strong>BSDI 2.0</strong></dt>

      <dd>Under BSDI 2.0 you can build Apache to support more
      descriptors by adding <code>-DFD_SETSIZE=nnn</code> to
      <code>EXTRA_CFLAGS</code> (where nnn is the number of
      descriptors you wish to support, keep it less than the hard
      limit). But it will run into trouble if more than
      approximately 240 Listen directives are used. This may be
      cured by rebuilding your kernel with a higher
      FD_SETSIZE.</dd>

      <dt><strong>FreeBSD 2.2, BSDI 2.1+</strong></dt>

      <dd>Similar to the BSDI 2.0 case, you should define
      <code>FD_SETSIZE</code> and rebuild. But the extra Listen
      limitation doesn't exist.</dd>

      <dt><strong>Linux</strong></dt>

      <dd>By default Linux has a kernel maximum of 256 open
      descriptors per process. There are several patches available
      for the 2.0.x series which raise this to 1024 and beyond, and
      you can find them in the "unofficial patches" section of <a
      href="http://www.linuxhq.com/">the Linux Information HQ</a>.
      None of these patches are perfect, and an entirely different
      approach is likely to be taken during the 2.1.x development.
      Applying these patches will raise the FD_SETSIZE used to
      compile all programs, and unless you rebuild all your
      libraries you should avoid running any other program with a
      soft descriptor limit above 256. As of this writing the
      patches available for increasing the number of descriptors do
      not take this into account. On a dedicated webserver you
      probably won't run into trouble.</dd>

      <dt><strong>Solaris through 2.5.1</strong></dt>

      <dd>Solaris has a kernel hard limit of 1024 (may be lower in
      earlier versions). But it has a limitation that files using
      the stdio library cannot have a descriptor above 255. Apache
      uses the stdio library for the ErrorLog directive. When you
      have more than approximately 110 virtual hosts (with an error
      log and an access log each) you will need to build Apache
      with <code>-DHIGH_SLACK_LINE=256</code> added to
      <code>EXTRA_CFLAGS</code>. You will be limited to
      approximately 240 error logs if you do this.</dd>

      <dt><strong>AIX</strong></dt>

      <dd>AIX version 3.2?? appears to have a hard limit of 128
      descriptors. End of story. Version 4.1.5 has a hard limit of
      2000.  Version 4.3.3 and 5.1 say
      <pre>
/*
 * Select uses bit masks of file descriptors.
 * These macros manipulate such bit fields.
 * FD_SETSIZE may be defined by the user to the maximum valued file
 * descriptor to be selected; the default here should be == OPEN_MAX
 */
#ifndef FD_SETSIZE
#define FD_SETSIZE     32767    /* must be == OPEN_MAX in <limits.h> */
#endif
</pre></dd>

      <dt><strong>SCO OpenServer</strong></dt>

      <dd>Edit the <code>/etc/conf/cf.d/stune</code> file or use
      <code>/etc/conf/cf.d/configure</code> choice 7 (User and
      Group configuration) and modify the <code>NOFILES</code>
      kernel parameter to a suitably higher value. SCO recommends a
      number between 60 and 11000, the default is 110. Relink and
      reboot, and the new number of descriptors will be
      available.</dd>

      <dt><strong>Compaq Tru64 UNIX/Digital UNIX/OSF</strong></dt>

      <dd>
        <ol>
          <li>Raise <code>open_max_soft</code> and
          <code>open_max_hard</code> to 4096 in the proc subsystem.
          Do a man on sysconfig, sysconfigdb, and
          sysconfigtab.</li>

          <li>Raise <code>max-vnodes</code> to a large number which
          is greater than the number of apache processes * 4096
          (Setting it to 250,000 should be good for most people).
          Do a man on sysconfig, sysconfigdb, and
          sysconfigtab.</li>

          <li>If you are using Tru64 5.0, 5.0A, or 5.1, define
          <code>NO_SLACK</code> to work around a bug in the OS.
          <code>CFLAGS="-DNO_SLACK" ./configure</code></li>
        </ol>
      </dd>

      <dt><strong>Others</strong></dt>

      <dd>If you have details on another operating system, please
      submit it through our <a
      href="http://httpd.apache.org/bug_report.html">Bug Report
      Page</a>.</dd>
    </dl>

    <p>In addition to the problems described above there are
    problems with many libraries that Apache uses. The most common
    example is the bind DNS resolver library that is used by pretty
    much every unix, which fails if it ends up with a descriptor
    above 256. We suspect there are other libraries that similar
    limitations. So the code as of 1.2.1 takes a defensive stance
    and tries to save descriptors less than 16 for use while
    processing each request. This is called the <em>low slack
    line</em>.</p>

    <p>Note that this shouldn't waste descriptors. If you really
    are pushing the limits and Apache can't get a descriptor above
    16 when it wants it, it will settle for one below 16.</p>

    <p>In extreme situations you may want to lower the low slack
    line, but you shouldn't ever need to. For example, lowering it
    can increase the limits 240 described above under Solaris and
    BSDI 2.0. But you'll play a delicate balancing game with the
    descriptors needed to serve a request. Should you want to play
    this game, the compile time parameter is
    <code>LOW_SLACK_LINE</code> and there's a tiny bit of
    documentation in the header file <code>httpd.h</code>.</p>

    <p>Finally, if you suspect that all this slack stuff is causing
    you problems, you can disable it. Add <code>-DNO_SLACK</code>
    to <code>EXTRA_CFLAGS</code> and rebuild. But please report it
    to our <a href="http://httpd.apache.org/bug_report.html">Bug
    Report Page</a> so that we can investigate. 
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